I'm looking for the
u16 prom chip for CPU board of a Roland D20,
would anybody have any idea where i can get one?
Try contacting Roland or your official Roland Service Center first. Korg and Yamaha both have reputations for helping out with their products even when they are long out of warranty. Roland doesn't have that reputation as much, but you should still start by asking them.
If you can't get Roland to supply you with a chip, then look for forums where people who maintain old Roland gear hang out. It seems unlikely that anyone would stock ROM chips like this, so you are looking for someone who (a) has a D20 already, (b) knows how to pull and read the PROM chip and (c) has a new blank chip to burn the data to.
Finally, if you can't find a friendly helper in a forum, check eBay and similar. Again, you are unlikely to find someone who offers exactly what you want. Your best bet will be to find someone who deals in old synth ROMs and PROMs or someone who deals in old Roland parts, Ideally both.